(1) Field of the Invention
The present invention relates to a voltage converter for converting the electric power of a high voltage battery, equipped in an electric vehicle, to a low voltage and charging a low voltage battery.
(2) Description of the Related Art
Electric vehicles including a hybrid electric vehicle are typically equipped with a vehicle drive battery (high voltage battery) for storing a DC power of relatively high voltage (e.g. about 500 V), an inverter for converting the DC power of the vehicle drive battery to AC power, and a vehicle drive motor that is driven by the AC power converted by the inverter. By rotating driving wheels connected to the output shaft of the vehicle drive motor so that power can be transferred, the vehicle is caused to travel.
In addition to the vehicle drive battery, electric vehicles are further equipped with an electric equipment battery (low voltage battery) for storing a DC power that has a relatively low voltage (e.g. 24 V or 12 V). The electric equipment battery is used to operate vehicle-mounted electric equipment, which includes lamp equipment (such as head lamps, stop lamps, etc.), air-conditioning equipment (such as an air-conditioning compressor, capacitors, etc.), audio equipment (such as a car stereo set, etc.), steering and brake equipment (such as an electric power assisted steering wheel, brake vacuum pumps, etc.), control equipment (such as various electronic control units, etc.), and so forth.
The electric equipment battery is connected with the vehicle drive battery via a voltage converter (DC/DC converter), so that the electric power of high voltage from the vehicle drive battery is converted to a low voltage by the voltage converter and stored in the electric equipment battery.
To prevent excessive rush current from entering an electric system, such electric vehicles or hybrid electric vehicles perform a rush current suppression process, called precharge control, at the time of starting up the high voltage equipment (i.e., the vehicle drive motor).
This precharge control will hereinafter be described in detail. A switch (main contactor) is interposed between the vehicle drive battery and the inverter to break an electrical connection with the vehicle drive battery. To suppress a variation in the voltage supplied from the vehicle drive battery, a capacitor is provided at the input terminal of the inverter and smoothes an input voltage and then converts DC power to AC power.
However, if a direct current from the vehicle drive battery is input to the inverter via the main contactor, an extremely high current (rush current) will flow into the aforementioned capacitor in a short time and therefore damage, such as the contact of the main contactor being melted, will be caused.
To avoid such damage, a switch (precharge contactor) and a resistor are arranged in parallel with the main contactor so that, before the main contactor is made on, the precharge contactor is made on. In this arrangement, the capacitor at the input terminal is gradually charged and the voltage of the capacitor rises, so that a voltage difference between the vehicle drive battery and the capacitor (inverter) becomes small. Thereafter, the main contactor is made on. This control (precharge control) can prevent excessive rush current from being generated when the main contactor is made on, thereby avoiding the occurrence of damage to the main contactor.
FIG. 4 shows the electric circuit of the power source unit of a conventional electric vehicle. As shown in the figure, a vehicle drive motor 109 is connected with a vehicle drive battery 102 through an inverter 104. An electric equipment battery 103 is connected to the vehicle drive battery 102 through a DC/DC converter 101 in parallel with the inverter 104. The electric equipment battery 103 is further connected to vehicle-mounted electric equipment 110.
Between (1) the vehicle drive battery 102 and (2) the inverter 104 and DC/DC converter 101, a main contactor 106 and a precharge contactor 107 are interposed in parallel in order to break an electrical connection between them. A resistor 108 is arranged in series with the precharge contactor 107. Also, a capacitor 105 is arranged at the input terminal of the inverter 104.
Therefore, in the case where the vehicle drive motor 109 is actuated in starting the vehicle, the precharge contactor 107 is first made on so that the capacitor 105 is gradually charged. If the capacitor 105 is sufficiently charged, and a voltage difference between the vehicle drive battery 102 and the capacitor 105 becomes small enough, the main contactor 106 is made on.
If the main contactor 106 is made on, the electric power of the vehicle drive battery 102 is supplied to the vehicle drive motor 109 through the inverter 104, and driving wheels connected to the vehicle drive motor 109 are rotated. On the other hand, the electric power of the vehicle drive battery 102 is converted to a low voltage through the DC/DC converter 101 and stored in the electric equipment battery 103. The supply of electric power from the electric equipment battery 103 causes the vehicle-mounted electric equipment 110 to operate.
Note that the DC/DC converter 101 is further connected with a control power source. If a voltage is input from the control power source to the DC/DC converter 101 and also a voltage (operating voltage) in a normal range is input from the vehicle drive battery 102 (which is a charging power source) to the DC/DC converter 101, it begins to operate.
FIG. 5A shows a rise in voltage of the capacitor 105 at the time of precharge control. As shown in the figure, in the case where the vehicle drive motor 109 is actuated in starting the electric vehicle, the precharge contactor 107 is first made on at time T1, so that precharge control is started. The capacitor 105 is gradually charged and rises in voltage. Thereafter, if the voltage difference between the vehicle drive battery 102 and the capacitor 105 becomes sufficiently small (e.g. about 30 V) at time T2, the precharge control is concluded and the main contactor 106 is made on. If the main contactor 106 is made on, the voltage of the capacitor 105 rises steeply and becomes equal to the voltage of the vehicle drive battery 102 at time T3. If the voltage of the capacitor 105 reaches the operating voltage (e.g. 480 V) of the DC/DC converter 101 during the time interval of T2 to T3, the DC/DC converter 101 is operated and converts the high voltage power of the vehicle drive motor 102 to a low voltage and supplies electric power to the electric equipment battery 103.
The conclusion of the precharge control can be judged by whether the voltage difference between the vehicle drive battery 102 and the capacitor 105 is a predetermined value or less, as described above. The conclusion can also be judged by whether the lapse of time from the start of the precharge control reaches a predetermined period of time, or whether the voltage rise rate of the inverter 104 (capacitor 105) is a predetermined rate or less.
In the case where the conclusion of the precharge control is judged based on a voltage difference, a voltage rise rate, etc., the judgment is normally made as shown in FIG. 6. In the example of FIG. 6, the precharge conclusion is judged by whether the aforementioned voltage difference is a predetermined value or less.
As shown in FIG. 6, if precharge control is started, in step S01 the precharge contactor 107 is made on and the precharge control advances to step S02. In step S02, the difference between the voltage Vb of the vehicle drive battery 102 and the voltage Vinv of the inverter 104 is compared with a predetermined value. When the difference is the predetermined value or less, precharge is judged to have been concluded and the precharge control advances to step S03. Then, in step S03, the main contactor 106 is made on, while the precharge contactor 107 is made off.
In step S02, if the difference between the vehicle drive battery voltage Vb and the inverter voltage Vinv is greater than the predetermined value, the precharge control advances to step S04. In step S04, the lapse of time from the start of the precharge control is compared with a predetermined period of time (5 seconds in the example shown in FIG. 6). When the lapse of time is within the predetermined period of time, the precharge control returns to step S02. Also, in step S04, when the lapse of time is the predetermined period of time or greater, the circuit is judged to have some fault and the precharge control advances to step S05. In step S05 an error display is performed, and since the lapse of time exceeds the predetermined period of time, the precharge control is stopped.
However, in the aforementioned prior art, it sometimes takes a much longer time than normal to start and conclude the precharge control. In that case, before the voltage of the capacitor 105 rises sufficiently, the precharge control exceeds a predetermined period of time. As a result, trouble such as the start-up failure of the DC/DC converter 101 sometimes occurs.
Because of this, the inventors have made various investigations with respect to this phenomenon and found the following fact. That is, the DC/DC converter 101 sometimes begins to operate during precharge control. This can cause the aforementioned trouble.
If the DC/DC converter 101 begins to operate during precharge control, electric power will be supplied to the electric equipment battery 103 through the DC/DC converter 101. As a result, the electric power that is supplied to the capacitor 105 is reduced and it takes time to charge the capacitor 105. In addition, when a demand for electric power on the load side (vehicle-mounted electric equipment 110, etc.) is great, a great variation in voltage occurs and during precharge control the electric power charged in the capacitor 105 flows to the load side, so that the voltage of the capacitor 105 is sometimes reduced.
Because of this, the capacitor 105 is not charged quickly and the precharge control takes time. Before the voltage of the capacitor 105 rises sufficiently, the precharge control exceeds a predetermined period of time. As a result, trouble such as the start-up failure of the vehicle drive motor 109 is caused.
In addition, as described above, if the DC/DC converter 101 operates during precharge, a rise in the voltage of the capacitor 105 becomes gentle. This can cause misjudgment if the conclusion of the precharge control is judged based on the condition of a lapse of time, a voltage rise rate, or the like. Even when the voltage of the capacity 105 does not rise sufficiently and the original precharge conclusion condition is not satisfied, there is a possibility that the main contactor 106 will be made on and excessive rush current will be generated.
Hence, the inventors have made further investigations with respect to the cause of the DC/DC converter 101 operating during precharge control and found that the cause lies in that, depending on the charged condition of the vehicle drive battery 102, its voltage varies up and down. This cause will hereinafter be described in further detail.
The operating voltage of the DC/DC converter 101 is set to a slightly lower value than the lower limit value of the normal voltage of the vehicle drive battery 102, based on the normal voltage range of the vehicle drive battery 102.
However, the voltage of the vehicle drive battery 102 sometimes rises to about the upper limit value of the normal voltage when it is fully charged. For instance, when the normal voltage range is 485 to 520 V, the actual voltage of the vehicle drive battery 102 sometimes reaches 520 V, as shown in FIG. 5B. In this case, during precharge control, the voltage on the inverter side reaches the operating voltage (480 V) of the DC/DC converter 101 before the voltage difference between the vehicle drive battery side and the inverter (capacitor) side becomes sufficiently small. This causes the DC/DC converter 101 to operate.
To overcome such trouble, if the operating voltage of the DC/DC converter 101 is set to a higher voltage side (e.g. 500 V), when the voltage of the vehicle drive battery 102 is close to the lower limit value of the normal voltage range there is a possibility that even if the main contactor 106 is made on after the conclusion of the precharge control, the DC/DC converter 101 will not operate.